Resize Related Post Thumbnails Span Width

I’m using JP 3.5.3 and have a question on customizing the display of related post thumbnails. From what I’ve read, I understand the images are resized via Photon. I’d really like to increase the size of my images and have them span the width of the div (making them more prominent on the page – I have a photography blog).

I’ve added the following to my functions.php file to customize the look: